Significance: Strabismus, misalignments of the visual axes, is prevalent in the US and usually treated surgically. Misdiagnoses and sub-optimal outcomes are predictable consequences of inadequate understanding of extraocular anatomy and oculomotor control. Our overall aim is to develop a quantitative, physiologically realistic understanding of extraocular biomechanics, applied to diagnosis and treatment of strabismus. We previously showed that distributed midorbital connective and smooth muscle tissues function as rectus extraocular muscle (EOM) pulley, crucial to normal ocular kinematics. Recent studies suggest that the global lamina of each EOM rotates the eye, whereas the orbital lamina translates the pulley that determines the global lamina's functional origin. More complete characterization of extraocular tissue architecture should reveal other mechanically significant structures, also not easily discriminable because distributed. These findings, and out direct physiologic muscle force measurements have cast doubt on the classic and fundamental oculomotor concept of the Final Common Path. Validation and development of these ideas will continue to have broad impact on laboratory studies of ocular motility and on diagnosis and treatment of strabismus. Studies: We will: (01) test predictions of differential contraction of global and orbital EOM laminae and movements of connective tissue pulleys with conjugate and vergence eye movements, using implanted gold microspheres and digital X-ray imaging in alert monkeys; (2) characterize extraocular tissue architecture by reconstructing multiple interlaced immunohistochemically stained thin serial sections of cadaveric human and monkey orbits; (3) test the Active Pulley Hypothesis, a challenge to the classic notion of the Final Common Path, using physiologic muscle force measurement in alert monkeys, and (4) develop scientifically and clinically useful biomechanical modeling tools that reflect current physiologic findings and hypotheses.